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JiB on GM’ing: Armor Class as Damage Reduction in Pathfinder

There has been a lot of discussion lately about the merits, or lack thereof, of armor class (and by extension hit points) in the d20 game. Armor class as an abstraction was done for a very simple reason, it was fast. It didn’t take a lot to figure out whether an attack was successful or not or whether that attack did damage. I do not disagree that it was possibly an over simplification and it is certainly wildly inaccurate. To be blunt if accuracy is what you want play GURPS. The d20 construct was never intended to be an accurate representation of combat or of much of anything really. Interestingly though despite the undeniable fact that there are more accurate game systems available d20 in its many versions and forms is still a popular and viable game system while some others have fallen into what can best be described as a niche existence. Part of the reason for that popularity is that we all know it. D&D is ubiquitous in the gaming world.


Fortunately for those of us who like the d20 construct but would like something a little more accurate the wonderful folks at Paizo have provided an alternative to armor class. The Advanced Combat Guide provides variant rules for armor class and hit points. The purpose of this JiB on GM’ing article is to explore those rules a bit.


First off a bit of explanation


Though we all have used armor class over the years it is useful to have a common understanding and so, “Armor class is an expression of how hard a character is to harm and is a product of the character’s dexterity plus any defenses they may have. Namely armor and shields.” This is my working definition of armor class for our purposes. I’m not going to concern our discussion with magical defenses because all they do is alter the numbers. They do not alter the basic equation. Since we brought up the concept of an equation here it is:


Armor Class = 10 + Armor Modifier + Shield Modifier + Dexterity Modifier


Which means that a character with a 14 Dex (+3), and wearing chainmail (+5) and carrying a heavy shield (+2) is going to end up with:


20 = 10 + 5 + 3 + 2


Which means that a monster with an attack bonus of +7 will need to roll a 13 or better on the die to hit. Which brings us to the math portion of our program. The monster will be successful approximately 35% of the time.


.35 = 7/20


Every shift of +/- 1 on either the character’s armor class or the monsters attack is a 5% shift one way or the other. Admittedly not very accurate, but easy to work with.


So what about armor as damage reduction


If we are to use armor as damage reduction in d20 it will need to still work with all of the magic items and spells that affect armor class. Fortunately the folks at Paizo are smart because in chapter 5 (Variant Rules) of the Ultimate Combat Guide they have an answer.


First throw out the concept of armor class alltogether. Replace it with a Defense score. Semantics but sure why not.


“In this alternative system, a creature does not have an Armor Class (AC); it instead has a Defense score. Defense is similar to touch AC in the standard Pathfinder Roleplaying Game rules, but it also adds the shield bonus (including any enhancement bonus to the shield), and any enhancement bonus to armor.” UCG p.191


Defense = 10 + shield bonus + Dexterity modifier + other modifiers (including armor’s enhancement bonus but not armor bonus or natural armor bonus)


Which means that our example character would end up with a Defense of 15


15 = 10 + 3 + 2


This leaves our character 25% more likely to be “touched” by an attack than if we were using armor class. But we’re not done yet.


Because of his chainmail our hero receives dr/5 armor so he gets to ignore the first 5 points of most attacks. Note that some creatures and some attacks bypass dr.


So let’s try an example (Oh quit complaining you knew it was coming)


Our hero is fighting a goblin and the goblin attacks with a cleaver (Why the goblin has a cleaver is anyone’s guess but he does.) For our purposes we’ll treat his cleaver as a dagger.


The goblin rolls his attack and gets a 17. If we were using AC the attack would miss and we would be done. But using armor as dr the 17 touches our hero so the goblin rolls damage and gets a 4. The four is less than the dr of his armor so the attack has no effect. The net result is the same. In aggregate (and somewhat anecdotally) it seems that attacks are somewhat more likely to do damage than with armor class but that the overall amount of damage is actually reduced. Which leads into the next JiB on GM’ing article, “Wounds and Vigor vs Hit Points.”




Based on the tests that I have run the results come out about the same either way. It is a bit more work to do armor as dr but not onerously so. Whether it is a better representation than armor class is a question I leave for the reader to answer. Personally I like it and am implementing the variant rules for the Pathfinder games that I run in the future.


Next I will take a look at Wounds and Vigor vs Hit Points.






It speeds up combat to roll the attack and the damage in the same cast so for the goblin attack above I would cast a d20 and a d4 at the same time. I tend to color group dice so if I had multiple attacks each attack has a color of its own.

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9 Responses to "JiB on GM’ing: Armor Class as Damage Reduction in Pathfinder"

  1. StuNo Gravatar says:


    A couple questions

    How does this alternate system take leveling into account, with increasing amounts of damage and hit points? Or does it?

    Does it offer some sort of conversion for monster stats so you can separate the damage reduction from monster AC? It would be easy to figure out humanoid opponents where armor is specified in the stats, but do they give you a rule of thumb for other monsters, like dragons, beholders, etc?

  2. rabaliasNo Gravatar says:

    Sure, this will work with low-level monsters. For high level monsters that do a lot of damage, you’ll see them hitting (a lot) more frequently but with a negligible effect on the amount of damage they do.

    (Which is fine… it just means the balance of power has shifted slightly, and badass monsters become more badass.)

    Actually, that may just be an extension of Stu’s question. I guess as you level up this will mean armour becomes increasingly unimportant, as the amounts of damage dealt grow faster than the defensive effects of armour…?

    I guess you could also argue that this makes things a tad more complex , rules lookup-wise. The advantage of the AC system is that you’ve got your AC, your touch AC and your flat-footed AC all available on your sheet without the need to calculate it each time, so the armour itself never gets looked up – it’s already factored in. Now you’ll have to look up the appropriate AC and the DR of the armour. (That’s assuming you’re the kind of person who finds these things hard to remember, of course.)

    Anyway, everyone’s a critic. My question would be, why include the enhancement bonus for magical armour in the defence score rather than the DR? Logically it feels like magical armour should be more about stopping damage than preventing hits? And in fact, could you alleviate the “badass monster” effect by saying that DR benefits from enhancement obey a non-linear rule, such that +1 armour gives DR +1, while +2 armour gives DR +3 (=1+2), +3 armour gives DR +6 (1+2+3) and so on? That might replicate the ability to take on bigger and bigger monsters that you see in high-level characters.

  3. JazzIsBluesNo Gravatar says:

    The numbers I used were from low level characters for the purpose of simplicity but you raise very valid points. Everyone bear in mind this is not my work I am simply reporting what Paizo published in the Ultimate Combat Guide. There is a very significant other part of this which is the variant rules for hit points. Namely Vigor and Wounds That is the next topic I intend to cover, and then I will do an exploration of leveling up characters and what needs to be done with monsters.

    If one uses Hero Lab ( it does the conversions for you which is really nice and handy.

    Thank you for the commentary. Very valid points that I will look into.

    Thank you,


  4. rabaliasNo Gravatar says:

    I look forward to the next installment!

  5. Philo PharynxNo Gravatar says:

    It’s hard to make such a huge structural change to the game without having lots of side effects, intended and unintended. For examples:
    Tthe guy in chainmail can effectively ignore a 12 strength goblin with a dagger. They will only do damage on a critical hit.
    Items that penetrate damage reduction are incredibly effective.
    Feats and items that add to defense are either nearly pointless (He now only has 65% chance to hit me) or hugely effective when you get a lot of stacking (he only has a 20% chance to hit me, and I get DR).
    Concealment and other things that grant miss chance are much more effective.
    You have to judge things like “does a barbarian’s DR stack with his armor?” If it does, then you increase the number of foes that they can ignore. If not, you nerf a significant class feature.
    Two weapon fighting becomes much less useful.
    BAB progression will eventually mean that most people will be hit all the time. (one answer is to change the 1, 3/4, 1/2 progression to 3/4, 1/2, 1/4)

    As you mention, adding wounds and vitality changes some of these. But it also adds it’s own set of complications. Like one-hit kills.

  6. heromedelNo Gravatar says:

    This wasn’t invented by paizo. There are rules for this in many sources before path finder for example in the Unearthed Arcana

  7. Jim PuderNo Gravatar says:

    Someone already beat me to the punch on this, but let me spell it out a bit more granularly.

    If you convert armor to DR, once you crosee, oh, about 8th level, you will get to either create new and exciting characters very frequently or learn to run the hell away from combat. For example, our party of 6 brave 7th level adventurers get into a slugfest with a Llamia. This creature hits, on average, for about 25 pts of damage per attack. So, if I am now easier to hit, and my elven chain (+1) grants me let’s say 7 points of DR, that means on average, I take 18 per attack. Significantly more frequently. Unless everyone’s armor is adamantine, that’s pretty bloody for the party. As an aside, this combat happened to our party, and I took a significantly higher than average hit and took 51 points of damage, leaving me with 13. I was fresh right before the strike. Had we not had a cleric in our group, and had the fighter not shown up when he did, I would have been toast. And I am a fairly tough Magus build. So imagine how much more frequently I could expect to get hit, and how much more damage I would take. No thanks, this method doesn’t appeal to me one little bit.

  8. themetalpopeNo Gravatar says:

    I think this was long overdue: armor should NEVER EVER decrease your chances of getting hit, it should ALWAYS subtract from damage. Old AC (and HP) is an archaic conept that should never have made it into the 21st century. The optional rules alleviate this and the problems that come with monsters at higher Levels dealing more damage should be easily handled by a capable Game Master. Also, don’t forget this makes most monsters easier to hit too, so in many cases the PCs will actually have the advantage because they’ll often outnumber the high-powered adversaries.

    1. HedisusNo Gravatar says:

      The argument of Armour as AC vs Armour as DR is a useless one. Both accomplish the exact same effect (providing the players a way to try and keep their character’s lifeblood inside their imaginary bodies) with a different flavour to them.

      Saying one is better than the other is like how some people say chocolate ice cream is better than strawberry, and others say the opposite. It’s all in your head about which you believe is better, since there is NO SINGLE CORRECT answer for everyone.
      Some people can and do like both flavours, and ALL of those viewpoints (Choco- supremists, Berry-supremists, and those who say both suck) ARE ALL correct answers.

      The only wrong answer is those Neapolitan weirdoes mixing it all together in one horrid, depraved mess of blasphemy (*Spoiler* J/K with this last paragraph, since there will inevitably be that one guy/girl/other who won’t get that it’s a joke /end*Spoiler*)

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